This invention relates generally to the drainage of aqueous humor from an eye to relieve the elevated eye pressure characteristic of glaucoma. More specifically, the present invention relates to an implantable glaucoma shunt device and related method, which prevents excess drainage of aqueous humor during the early post-operative period, and yet permits sufficient fluid flow from the eye during long term use.
Aqueous humor is continuously produced by the ciliary body in the posterior chamber of the eye, and from there it flows into the anterior chamber. In order to maintain relatively constant intraocular pressure, aqueous humor must be drained away continuously. It passes through the trabecular meshwork and into the canal of Schlemm, before draining into the veins leaving the eye.
Normal intraocular pressure typically ranges from about 15.+-.3 mm Hg., but may be up to 21 mm Hg. Pressures substantially above this range are considered abnormally high. Chronically elevated intraocular pressure (resulting, for example, from a defect in intraocular drainage) can give rise to glaucoma. Glaucoma can cause irreversible damage to certain structures of the eye, including the retina, and is the leading cause of blindness in the United States.
There are many types and causes of glaucoma. Optimal treatment depends on the patient's form of glaucoma. As a rule, the damage caused by glaucoma cannot be reversed. The goal, therefore, of treatment is to prevent further damage and to preserve existing vision.
Glaucoma can often be controlled with medical therapy, typically through topical medications (eyedrops) and systemic medications (pills). Medical therapy either decreases the rate of formation of aqueous humor, or increases its outflow from the anterior chamber. Potential problems with medical treatment include side effects, inadequate control of the intraocular pressure, and poor patient compliance.
If the maximum-tolerated dose of medication fails to control the intraocular pressure, then laser trabeculoplasty or filtering surgery is usually indicated. These procedures seek to increase the rate of outflow of aqueous humor. Another type of surgical procedure seeks to reduce the rate of formation of aqueous humor, by destroying the tissue where it is created. These cyclodestructive procedures are typically indicated only after filtering surgery has failed. If such filtering surgery has failed to control the intraocular pressure, or if the patient has a poor prognosis for filtering surgery, implantation of a glaucoma shunt may be indicated.
Glaucoma shunts typically drain aqueous humor from the anterior chamber of the eye to a fibrous capsule (bleb) which forms around a collecting device placed on the posterior portion of the globe of the eye. Aqueous humor is then resorbed into the vascular system. Glaucoma shunts typically consist of a silicone elastomer catheter which is inserted into the anterior chamber, and which connects to an episcleral plate or an encircling band. Episcleral plates are commonly made of silicone elastomer, polypropylene or acrylic materials.
Glaucoma shunt surgery is subject to a number of common complications. In the early post-operative period, excess drainage of aqueous humor from the anterior chamber can cause low intraocular pressure and shallow anterior chamber depth. This can lead to choroidal detachment or hemorrhage. The low intraocular pressure is typically alleviated as the fibrous capsule forms around the posterior plate or encircling band. In the long-term, excess fibrosis of the capsule obstructs the flow of aqueous humor from the bleb into the vascular system. This causes an increase in the intraocular pressure, and results in clinical failure of the device.
Accordingly, there has been a need for a novel glaucoma shunt which, in the immediate post-operative period, can provide resistance to flow to maintain a desired intraocular pressure. Such a shunt should also interrupt the formation of a dense capsule around the shunt to ensure the continued resorption of aqueous humor by the vascular system, and help prevent long-term intraocular pressure elevation. Further a glaucoma shunt is needed which accomplishes the desired function and is easy to manufacture and use, and which provides the desirable shunting functions reliably over an extended period of time. The present invention fulfills these needs and provides other related advantages.